1. Field of the Invention
Generally, the invention pertains to the field of vacuum thin film deposition onto substrates using ion sources and modifying properties of substrates by ion beam treatment. Specifically, the invention focuses upon a new and improved method and apparatus for providing the supply of electrons onto a thin film substrate for neutralization of electric charge brought onto the surface of said substrate and other surfaces of the apparatus by the ion beam radiating from a closed drift ion source.
2. Description of the Prior Art
When a substrate is exposed to an ion beam, a need arises to close the electrical current path, or in another words to neutralize the electrical charge brought with the ions onto the substrate surface as well as onto other surfaces of the apparatus. This need is especially apparent for insulating substrates, but even conductive substrates as well as other surfaces of the apparatus frequently have poorly conducting inclusions or thin films on them. If those films or inclusions are not discharged, the accumulated charge may lead to an electrical breakdown and an onset of an arc that may introduce defects at the substrate surface. Although similar charging problems exist in other vacuum processing fields, such as in reactive magnetron sputtering, the ion beam processing is an entirely different technology. It operates on different principles, at different voltage levels, different polarities, and even with charged particles hitting different surfaces and so is not thought of as analogous. Ion beam etching of silicon oxide is one example of an ion beam process that is addressed by present invention. The invention specifically pertains to closed drift ion sources, such as the LIS and MCIS series manufactured by Advanced Energy Industries of Fort Collins, Colo. These closed drift ion sources are quickly finding new applications due to their rugged design and low maintenance. Solving neutralization problems for these sources opens markets for these types of supplies.
Most ion sources incorporate some kind of an electron emitter device, commonly referred to as a neutralizer to supply electrons onto a substrate surface as taught, for instance, in a reference book, entitled “Handbook of Ion Beam Processing Technology” edited by Jerome J. Cuomo, Stephen M. Rossnagel and Harold R. Kaufman (Noyes Publications), hereby incorporated by reference. The electron emitter frequently doubles as a neutralization device, or a second emitter sometimes is used specifically for neutralization.
Two basic types of electron emitters are being commonly used, a hot filament and a hollow cathode. A significant problem with the use of hot filament thermionic electron emitters is that the operational lifetime of the emitters can be very limited, often less than 100 hours. This may be especially true when reactive gases, such as oxygen, are present in the ion source. Similarly, hollow cathode electron emitters can have a lifetime of about 1000 hours as disclosed by U.S. Pat. Nos. 3,156,090; 3,913,320; 3,952,228; 3,956,666; and 3,969,646, each hereby incorporated by reference. An additional problem with an electron emitter neutralizer is that it can introduce additional complexity to the system, and perhaps, a need to align the position of the neutralizer so that it has good coupling with the plasma but does not stand in the way of the ion beam. Yet another additional problem with an electron emitter neutralizer can be a non-uniformity that it may introduce onto the process, especially with wide aperture beams. To mitigate this non-uniformity several neutralizers have sometimes been used.
Additionally, U.S. Pat. No. 5,973,447 issued to L. J. Mahoney et al disclosed an ion source that utilizes a separate self-sustaining cathode 74 with dc power sources 84 and 86 to generate electrons. This type of electron emitter is typical of the complexity that some designs add to the process.
Closed-drift ion sources, like the LIS series and the MCIS series manufactured by Advanced Energy Industries of Fort Collins Colo., do not require an electron emitter for their operation. It has been discovered that sufficient electron emission can be obtained by connecting the power source directly to the anode of the ion source using the ion source housing as the cathode. This can greatly expand the application range by making the source design reliable and rugged and having long maintenance-free operation time. However, in certain thin film applications that are not tolerant to ion beam charging there can be a great need for a way to neutralize the substrate surface without compromising the advantages of the ion source design.
For grid ion sources a different method of thin film neutralization, not requiring a hot electron emitter, was proposed by D. Korzec, T. Kebler, H. M. Keller and J. Engelmann in “Filamentless Neutralization of Broad Ion Beams”, in the Journal of Vacuum Science and Technology, B9 (1991), pgs 3084-3089. According to this publication, a bipolar pulsing of grid voltages can make the ion source work as an electron source during the reverse polarity part of every pulse. Since thin films can have high capacitance, the voltage on the surface may not change significantly during the pulse so the film surface can be kept at low potential if the pulse frequency is high enough. This method did not seem to be applicable to closed drift ion sources because a closed drift source cannot sustain its discharge with reversed bias voltage. The reason for this is that magnetic field lines in the closed drift ion source usually must have both ends terminating at the cathode so electrons cannot arrive from cathode to anode along the magnetic field lines. However, almost always all magnetic field lines originating at the anode terminate at the cathode, so reverse bias could force electrons to the positive side of the source before they can make any ionization. Moreover, a shortened duty cycle appeared to cause reduction of the ion source throughput and this alone could make this method commercially unattractive.